The present invention relates to a drawing triangle or a protractor, in particular for blackboard use.
Known accessories of this type for blackboards consist normally of several parts, namely plane members with different shape--such as triangular, circular or semi-circular plane members provided with handles. Such a construction may cause damage, in form of breaks between the plane member and the handle, or local damage to edges and points caused by pressure.
U.S. Pat. No. 1,225,464 discloses a spatial, closed pyramid-shaped drawing implement, which has four flat side surfaces having their central parts recessed in such a manner that drawing straight edges are formed along the outer side edges of all four flat sides. In other words this known drawing implement has up to four usable sides and is not suitable for use on blackboards.